One technology for after-treatment of engine exhaust utilizes selective catalytic reduction (SCR) to enable certain chemical reactions to occur between NOx (nitrogen oxides) in the exhaust and ammonia (NH3). NH3 is introduced into an engine exhaust system upstream of an SCR device by injecting urea into an exhaust pathway, or is generated in an upstream catalyst. The urea entropically decomposes to NH3 under high temperature conditions. The SCR facilitates the reaction between NH3 and NOx to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventors herein, issues may arise upon injecting urea into the exhaust pathway. In one example, urea may poorly mix into the exhaust flow (e.g., a first portion of exhaust flow has a higher concentration of urea than a second portion of exhaust flow) which may lead to poor coating of the SCR and poor reactivity between emissions (e.g., NOx) and the SCR device. Additionally, overly mixing and agitating the urea in the exhaust can likewise cause issues, such as increased deposits.
Attempts to address insufficient mixing include introducing a mixing device downstream of a urea injector and upstream of the SCR device such that urea dispersion with exhaust gas may be more homogenous. Other attempts to address urea mixing include a stationary mixing apparatus. One example approach is shown by Cho et al. in U.S. 2013/0104531. Therein, a static mixer is located in an exhaust passage downstream of an external tube for injecting urea. The exhaust gas flows through the exhaust passage and merges with a urea injection before flowing through the static mixer.
However, the inventors herein have recognized potential issues with such systems. As one example, the static mixer described above presents limited mixing capabilities due to a directionality of exhaust outflow through the mixer being unable to fully mix the urea and exhaust gas. The static mixer inside the exhaust passage also presents manufacturing and packaging constraints. Varying exhaust passage geometries demand an alteration in the manufacturing of the exhaust passage and/or static mixer for the mixer to tightly fit within the exhaust passage. Lastly, the static mixer may overly agitate the urea. Thus, deposits may form on surfaces of the mixer and/or downstream of the mixer (onto the SCR device, for example). These deposits may accumulate and hinder exhaust flow through the exhaust passage thereby increasing an exhaust backpressure.
The inventors herein have recognized the issues with the above approach and offer a system to at least partly address them. In one example, the issues described above may be addressed by a system for a hollow cylindrical selective catalytic reduction device located along an exhaust passage comprising a plurality of plates configured to rotate via a rotatable rod as exhaust gas flows through the device, and a shaft configured to flow urea into the device, where the urea is stored in a lower portion of the device. In this way, the SCR does not rely on a urea injector and/or a mixer.
As one example, the plates comprise of one or more SCR catalysts and divide an interior of the SCR into compartments, where the compartments are fluidly coupled to one another. Exhaust gas flows into a compartment and presses against a downstream plate to rotate the plates toward an outlet of the SCR. The plates rotate through the stored urea, where the urea may coat surfaces of the plates or the urea may vaporize as exhaust gas heats it by flowing through the plates toward the urea. By doing this, the urea is dispersed into a head space of the SCR, where a rotation of the plates may mix the urea with exhaust gas. Thus, an area for the urea to mix into is decreased, which may increase a dispersion efficiency without the use of a urea injector and/or mixer. This may decrease emissions without additional packaging constraints being introduced to the vehicle.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.